1. Field of the Invention
This invention relates to a method of superimposing data of a character or a mark, such as @, and .fwdarw., on video data, the video data including the superimposed data being subjected to a data compression processing in which the video data including the superimposed data within a block of pixels are extracted from one frame of the video data including the superimposed data and a data compression is effected at every block of pixels.
2. Description of the Prior Art
A prior art method of superimposing data of a character or a mark on video data in a digital VTR (video tape recorder) is known. In such a digital VTR, the band compression (data compression) of video data is carried out to reduce an amount of tape consumption. Therefore, an image of the superimposed character of a mark is affected by the band compression because a character or mark has steep edges.
FIG. 5 is a block diagram of an example of a video data compression unit 23 for effecting a video data compression processing. FIG. 6 is an illustration of a DCT coefficient block for showing the zigzag scanning operation carried out in the prior art video data compression unit shown in FIG. 5. FIGS. 5 and 6 are used in the embodiment of this invention also.
An extraction circuit 10 consecutively extracts data of a DCT (Discrete cosine Transform) block including 8.times.8 pixels from one frame of video data. The extracted data is supplied to a DCT circuit 1 (8.times.8 DCT). The DCT circuit I supplies 8.times.8 conversion coefficients obtained by the two-dimensional discrete cosine transforming to a linear quantizer 2. The linear quantizer 2 effects the linear quantizing with a quantizing matrix obtained by multiplying a reference quantizing matrix supplied from a quantizing matrix generator 3 (quantizing matrix) by a scaling factor S with a multiplier 4 to provide a dc conversion coefficient 2a and ac conversion coefficients 2b which are supplied to a one-dimensional predictive unit 5 and a zigzag scanner 7 respectively. The one-dimensional predictive unit 5 compresses an amount of data of the dc conversion coefficients 2a successively incoming from the linear quantizer 2 and supplies the compressed data to a first Huffman coder 6. The first Huffman coder 6 effects a Huffman coding and sends the result to a multiplexer 9.
The zigzag scanner 7 scans the ac conversion coefficient 2b zigzag from low to high spatial frequencies as shown in FIG. 6 and sends the result to a second Huffman coder 8. The second Huffman coder 8 produces Huffman codes from a run length of coefficients indicting "0" and from a coefficient of not "0" similarly and send them to a multiplexer 9. The multiplexer 9 multiplexes the output of the first Huffman coder 6 and the output of the second Huffman coder 8 to produce and send a multiplexed signal 9a to a transmission line (not shown). The multiplexed signal including the first and second Huffman codes is recorded on a magnetic tape (not shown).
The scaling factor S is provided for controlling an amount of codes included in the multiplexed signal 9a per one field to a constant; otherwise the amount of codes included in the multiplexed signal 9a will vary in accordance with input video data because a variable code length is used in the data compression mentioned above.
However, there is a problem that in the digital VTR effecting the video data compression in which video data within a block of pixels are extracted from one frame of the superimposed video data and data compression is effected at every block of pixels, the picture quality of an image around the character or mark is affected in accordance with a position or shape of a character.
More specifically, in the video band compression apparatus, one frame of a luminance signal is sampled at a matrix having 480 (vertical).times.720 (horizontal) pixels. The sampled data are divided into blocks, each having 8.times.8 pixels, that is, into 60 (vertical).times.90 (horizontal) blocks.
In such an image data compression apparatus using the DCT processing and in a general image data compression processing, a compression processing error (compression distortion) occurs at a steep edge or at a portion having a high frequency components. In this video data compression processing using the DCT, this compression processing error is referred to as "mosquito noise".
Generally, a character or a mark superimposed on an reproduced image in an analog or digital VTR has a size corresponding to 32 (vertical).times.16 (horizontal) pixels in the case of the above-mentioned parameters. FIGS. 7A to 7C are schematic illustration for showing a superimposed character through a prior art superimposing processing. When a character "5" is superimposed at a given position with respect to the matrix of the blocks (32.times.16) as shown in FIG. 7A, each block having 8.times.8 pixels, there arises an occasion the character "5" is superimposed spreads over 15 blocks as shown in FIG. 7A. FIG. 7B is a partial illustrations of FIG. 7A. In FIG. 7B, there are edges having low vertical and horizontal correlation in the block, so that the mosquito noise occurs in the block. Moreover, there is a tendency that the noise occurring around the character is visually prominent.